With recent more variety in eating habit, there has been an increasing demand for transportation of food fish alive. Up to the present time, low-temperature transportation of living fish in a dormant condition is widely spread. However, this method involves disadvantages of a low rate of survival after transferring the fish to an aquarium and a large consumption of electric power. On the other hand, in a transportation method of using a water tank in which fish are maintained in usual conditions, a large amount of oxygen is required and a density of fish in the tank is low. The density of fish during transportation from a fishing ground to a fishing port and during storage in a fishing port before delivery and rate of survival are directly linked with cost and are therefore important factors of transportation of living fish.
If a fish density is increased, the ammonia concentration in the water tank increases and, eventually, the fish perish from an increased concentration of a nitrite ion, due to a decomposition product of ammonia. To avoid this, a combined filtration method is employed, in which ammonia in a water tank is adsorbed onto activated carbon, coral sand, etc. and bacteria capable of degrading harmful substances are allowed to proliferate, but the problems have not yet come to satisfactory solutions.
Further, as the fish density increases, the rate of disease during fish keeping tends to be increased. For example, infection with infectious diseases, such as oodiniosis, trichodimiasis, lymphocystis disease, white spot disease, and tail rot, has arose a serious problem.
Various problems remaining unsolved as mentioned above, reliability in breeding, keeping, and transportation of fish is still low, and it has been demanded in various fields to find a solution as soon as possible.